1) Check value of  profile option :AR: Enable Debug Message Output, it should be set to Yes

2) Run the following select statement to determine where the log will write to :

select value from v$parameter where upper(name) = \'UTL_FILE_DIR\'

this returns something like :

VALUE
--------------------------------------------------------------------------------
/usr/tmp, /usr/tmp, /u01/oracle/mc3yd213/db/tech_st/11.1.0/appsutil/outbound/mc3yd213_bej301441, /usr/tmp

The above is just an example, the values for your database may be different,take note of the values as you will need to supply it later.

3) In the application, once you are in the form that you want to debug,click on :

Help - Examine

fill in window that pops up :
block = PARAMETER
field = AR_DEBUG_FLAG
value = FS

wherein you replace utl_file_dir with a value from the select above, and debug_file.log with an actual debug file name,
e.g. : FS /usr/tmp debug1.log

NOTE :
1) the spacing above is important, there is a space after FS and space before debug1.log
2) there is no LOV for field, you need to explicitly type in AR_DEBUG_FLAG
3) you need to type in the path exactly as shown in the select statement in #2 

navigate through the form as normal, once done, exit the form

in Unix : go to the utl_file_dir path you specified and you should find a file with the name you specified in #3

For More Information:

How Do You Create Debug Logs For AR Code [ID 205465.1]